1. Field of the Invention
The invention relates to a control system and control method for an internal combustion engine, which detect the concentration of SOx in exhaust gas with the use of a limiting current sensor that is able to detect the concentration of oxygen in exhaust gas from the internal combustion engine.
2. Description of Related Art
As a sensor that detects the concentration of oxygen in exhaust gas, there is known a limiting current sensor including a solid electrolyte, a pair of electrodes (an electrode pair formed of a measuring electrode and a reference electrode) and a diffusion-controlling layer (diffusion resistance layer). The solid electrolyte is able to conduct oxygen ions. The pair of electrodes are provided so as to sandwich the solid electrolyte. The diffusion-controlling layer is provided so as to cover the measuring electrode.
The principle of detecting the concentration of oxygen in exhaust gas with the use of the above sensor is as follows. Initially, when a voltage is applied to the electrode pair so that a predetermined potential difference is generated between the electrode pair, oxygen in exhaust gas is ionized at the measuring electrode. Oxygen ions pass through the solid electrolyte and migrate to the reference electrode, and then return into oxygen at the reference electrode through recombination. Migration of electrons due to the series of electrochemical reactions is output as a current from the electrode pair. Because the diffusion-controlling layer is controlling migration of oxygen to the measuring electrode, the magnitude of current that is output from the electrode pair is in a one-to-one correspondence with the concentration of oxygen in exhaust gas. Thus, when a current that is output from the electrode pair is measured, the concentration of oxygen in exhaust gas is identified (detected) on the basis of the measured current.
As one of such detecting devices, there is a detecting device that detects the concentration of a component other than oxygen (for example, the concentration of H2O, that is, humidity) by utilizing the fact that a molecular that is decomposed at the measuring electrode varies as the magnitude of voltage that is applied to the electrode pair varies. Specifically, the detecting device uses a sensor having two sets of electrode pair (an upstream-side electrode pair and a downstream-side electrode pair). The detecting device initially applies a voltage having a magnitude of decomposing oxygen to the upstream-side electrode pair, and emits oxygen in exhaust gas to the outside of the sensor. Subsequently, the detecting device applies a voltage having a magnitude of decomposing H2O to the downstream-side electrode pair, and measures a current that is output from the downstream-side electrode pair. The detecting device identifies the concentration of H2O in exhaust gas on the basis of the current. Thus, the detecting device is able to measure a current that is output because of H2O as distinguished from a current that is output because of oxygen, so it is possible to accurately detect the concentration of H2O (see, for example, Japanese Patent Application Publication No. 2-122255 (JP 2-122255 A)).
Hereinafter, the limiting current sensor that is able to detect the concentration of oxygen in exhaust gas is referred to as oxygen concentration sensor, the voltage that is applied to the electrode pair of the sensor is referred to as applied voltage to the sensor, and the current that is output from the electrode pair is referred to as output current of the sensor. The internal combustion engine is referred to as engine.
The amount of sulfur component contained in fuel for the engine is generally an extremely small amount. However, depending on a region, or the like, in which the engine is used, fuel having a relatively high concentration of sulfur may be used. In this case, sulfur oxides (SOx) that are produced during combustion can cause white smoke, or the like, of exhaust gas. Therefore, it is desired to measure the concentration of sulfur in fuel. The inventors of the invention have studied that the concentration of SOx in exhaust gas is detected as a parameter related to the concentration of sulfur in fuel.
Specifically, the inventors of the invention have studied whether it is possible to detect the concentration of SOx in exhaust gas by using the method employed in the above-described detecting device. As a result of the study, it has been found that, when a voltage having a magnitude of decomposing SOx (in other words, reducing SOx into sulfur) is applied to the electrode pair of the oxygen concentration sensor, not only SOx but also a component other than SOx in exhaust gas also decomposes, and it has been found that it is not easy to measure only an output current due to SOx separately. That is, with the method employed in the existing device, there is a case where it is difficult to accurately detect the concentration of SOx in exhaust gas.